Coevaporation of formamidinium lead iodide (FAPbI₃) is a promising route for the fabrication of highly efficient and scalable optoelectronic devices, such as perovskite solar cells. However, it poses experimental challenges in achieving stoichiometric FAPbI₃ films with a cubic structure (α-FAPbI₃). In this work, we show that undesired hexagonal phases of both PbI₂ and FAPbI₃ form during thermal evaporation, including the well-known 2H-FAPbI₃, which are detrimental for optoelectronic performance. We demonstrate the growth of α-FAPbI₃ at room temperature via thermal evaporation by depositing phosphonic acids (PAc) on substrates and subsequently coevaporating PbI₂ and formamidinium iodide. We use density-functional theory to develop a theoretical model to understand the relative growth energetics of the α and 2H phases of FAPbI₃ for different molecular interactions. Experiments and theory show that the presence of PAc molecules stabilizes the formation of α-FAPbI₃ in thin films when excess molecules are available to migrate during growth. This migration of molecules facilitates the continued presence of adsorbed organic precursors at the free surface throughout the evaporation, which lowers the growth energy of the α-FAPbI₃ phase. Our theoretical analyses of PAc molecule–molecule interactions show that ligands can form hydrogen bonding to reduce the migration rate of the molecules through the deposited film, limiting the effects on the crystal structure stabilization. Our results also show that the phase stabilization with molecules that migrate is long-lasting and resistant to moist air. These findings enable reliable formation and processing of α-FAPbI₃ films via vapor deposition.

中文翻译：

---

通过膦酸调整界面能量以生长和稳定热蒸发沉积的立方 FAPbI3


甲脒碘化铅（FAPbI ₃ ）的共蒸发是制造高效且可扩展的光电器件（例如钙钛矿太阳能电池）的一条有前途的途径。然而，它对实现具有立方结构的化学计量FAPbI ₃薄膜（α-FAPbI ₃ ）提出了实验挑战。在这项工作中，我们证明了 PbI ₂和 FAPbI ₃在热蒸发过程中形成了不需要的六方相，包括众所周知的 2H-FAPbI ₃ ，这对光电性能不利。我们通过在基板上沉积膦酸（PAc）并随后共蒸发 PbI ₂和甲脒碘化物，通过热蒸发证明了 α-FAPbI ₃在室温下的生长。我们使用密度泛函理论开发了一个理论模型，以了解不同分子相互作用下 FAPbI ₃的 α 相和 2H 相的相对生长能量。实验和理论表明，当过量分子在生长过程中可迁移时，PAc 分子的存在可以稳定薄膜中 α-FAPbI ₃的形成。这种分子迁移有利于在整个蒸发过程中吸附的有机前体在自由表面上持续存在，从而降低了 α-FAPbI ₃相的生长能。我们对 PAc 分子间相互作用的理论分析表明，配体可以形成氢键，以降低分子穿过沉积膜的迁移速率，限制对晶体结构稳定的影响。我们的结果还表明，迁移分子的相稳定是持久的并且耐潮湿空气。 这些发现使得能够通过气相沉积可靠地形成和加工 α-FAPbI ₃薄膜。